With a trend toward higher-density magnetic recording, spin-valve giant magneto-resistive effect heads have been used as read heads of HDDs (Hard Disk Drives), and, to the present, the improvement of the film structure has increased their reading output. A spin-valve giant magneto-resistive effect head has a laminated structure of antiferromagnetic layer/pinned layer/non-magnetic intermediate layer/free layer, in sequence. The magnetization of the pinned layer is fixed by an exchange coupling magnetic field generated at the interface between the antiferromagnetic film and the pinned layer, while the magnetization of the free layer is reversed by an external magnetic field, thus changing the relative magnetization directions in the pinned layer and the free layer. This causes a change in electrical resistance, whereby a magnetic field is detected. In this case, the applied current is parallel to the film surface. Such a mode that applies a current parallel to a film surface is generally called CIP (Current-in-Plane). In recent years, in order to further increase output, research and development have been conducted on TMR (Tunneling Magneto-resistive) heads and CPP (Current Perpendicular to Plane)-GMR (Giant Magneto-resistive) heads, wherein a current flows perpendicularly to the film surface. TMR heads develop magnetoresistance with the spin-dependent tunneling effect, and thus have high magnetoresistance change ratio (MR ratio). However, although the MR ratio is high, an insulating layer is required for a magnetic tunnel junction, and accordingly, the resistance-area product (RA) thereof is as high as several Ωμm2. Therefore, in case of a small element, the head resistance is high, resulting in poor high-frequency characteristics. TMR heads are thus disadvantageous in high-speed transmission.
As a CPP-GMR head, when a current is applied perpendicularly to the film surface in a structure as with the conventional CIP-GMR, the resulting MR ratio is low, and practical application thereof is thus difficult. In order to achieve higher MR ratio, research has been conducted on the application of a half metal to a ferromagnetic layer. Half metal is a metal in which only spin-up or spin-down electrons exist at the vicinity of Fermi surface. Such a metal makes a huge difference in mean free path between the spin-up conduction electrons and spin-down conduction electrons, possibly resulting in high MR ratio. J. Magn. Magn. Mater., 198-199, 55 (1999) (“Nonpatent Document 1”) discloses a CPP-GMR sensor containing a Heusler alloy, a kind of half metal. Although the MR ratio is not so high, about 8% at 4.2 K, the disclosure is significant in showing a possibility that Heusler alloys are applicable to a CPP-GMR element.
JP-A-2003-218428 (“Patent document 1”) discloses an invention that uses, as a material for a Heusler alloy, Co2MnZ (Z=Al, Si, Ga, Ge, Sn) for a CPP-GMR element. JP-A-2004-221526 (“Patent document 2”) discloses an invention that uses Co2(FexCr1-x)Al for a TMR element or CPP-GMR element. JP-A-2007-81126 (“Patent document 3”) discloses an invention that uses (CoPd)MnZ (Z=Sn, Ge, Si) and (CoX)MnSn (X=Rh, Ru, Ir) for CPP-GMR elements.
These CPP-GMR elements may have a dual spin-valve structure to have further increased MR ratio. However, they do not meet the demand for a small read gap for increasing the resolution of the magnetic head. Moreover, all the CPP-GMR elements disclosed in Patent Documents 1 to 3 are formed of metal films, thus having low RA, and therefore involve a problem in that sufficient output cannot be yielded unless the element size is considerably small to increase the head resistance.
As another structure, Japanese Patent No. 3,293,437 (“Patent Document 4”) suggests a CPP-GMR element having inserted therein a nonmagnetic film comprising a mixture of an insulating material and a conductive material. If a spin-valve structure has such a layer comprising a complex of an insulating material and a conductive material, the current perpendicular to the film surface flows preferentially through the conductive material in the nonmagnetic film, and accordingly, the RA and MR ratio can be increased. J. Appl. Phys., 97, 10c509 (2005) (“Nonpatent Document 2”) states that in a CPP-GMR element having a current confinement layer comprising AlCu, the MR ratio was 4.3% when RA=0.38 Ωμm2. However, in order to achieve a high head SNR, still higher MR ratio is required.
As mentioned above, the MR ratio of a spin-valve CPP-GMR head is still too small to achieve a next-generation recording density of a level of 300 Gbit/in2, and the sensitivity of the magneto-resistive effect element is also insufficient for this purpose. Moreover, because the recording density in the bit direction increases with an increase in the recording density, the shield gap has to be narrower to maintain high resolution.
Conventionally, CPP-GMR heads containing a Heusler alloy or like highly spin polarized material would require a thick highly spin polarized layer (generally 5 nm or more), so as to maintain the crystallinity of the highly spin polarized layer and obtain large spin-dependent bulk scattering. Otherwise, a dual spin-valve structure would be required to increase spin-dependent scattering. Whichever structure was employed, the total thickness of the read sensor would be large, and therefore, it was difficult to produce a CPP-GMR head ready for a small read gap. Moreover, it has been revealed that oxidation of a highly spin polarized material greatly degrades its characteristics, so simple combination with an oxide-containing current confinement layer was difficult.